Research on multiple object tracking has shown that we can track objects moving across the visual field with attention. Given that most studies used 2D stimuli, little is known about the characteristics of dynamic attention in depth. We examined how separation of objects across different depth planes affects tracking performance under free- and fixed-viewing conditions in two experiments. Stimuli were eight virtual tennis balls bouncing inside two virtual zones measuring 2.5 x 1.25 x 0.62 m (W x H x D) each, placed one above the other. There were two targets and two distracters in each zone. The task required tracking the target balls for 8 s. The speed of the balls was adjusted for each subject in a separate threshold procedure. Stimuli were displayed with binocular disparity on a large screen inside an immersive environment. In Experiment 1, 20 participants tracked targets located in two zones that were separated by 0, 0.62, 1.24, or 1.86m in depth. Participants' eye movements were not restricted. Results showed large variability across subjects and no effect of depth separation on performance. In Experiment 2, participants tracked targets while fixating in the center between the top and bottom zones located either at the same depth or separated by 1.86m. Performance was better (by ~6%) for the same-depth compared to the different depth condition in most subjects. Performance was also significantly better (by ~20%) in the lower compared to the upper visual field. The lower visual field advantage was also seen in two control conditions where participants tracked targets only in the top or in the bottom zone. These results are consistent with previous reports of better attentional resolution in the lower visual field. Distributing attention across depth planes appears to have a small detrimental effect on tracking performance in fixed-viewing mode.